1. Field of the Invention
The present invention relates to optical waveguides based on polyimides and to a method for the preparation thereof.
2. Description of the Related Art
The implementation of optical communication systems, made possible through the development of low loss optical fibers, is associated with the need for the development of various components useful in such field. Particularly, there is considerable demand for optical circuits and components thereof which support high throughput of communication signals, especially for optical waveguides applicable to such systems.
An optical waveguide comprises an optical core material embedded in a cladding material.
Among the various known polymers, polyimides provide very high resistance to heat and are employed in the field of electronics as optical waveguides. The use of polyimides in the optical communication fields provides two main advantages: excellent transparency in the visible and near infrared region and a refractive index variable in a wide range.
U.S. Pat. Nos. 5,572,619 and 5,598,501 discloses describe polyimide optical waveguides and methods for manufacturing them, in particular disclose a polyimide optical waveguide comprising a core made of a polyimide whose refractive index is controlled to a predetermined value by electron beam irradiation. Manufacturing is performed by a method of comprising the steps of:                forming a first polyimide layer on a substrate;        forming a core layer having a predetermined refractive index by irradiating said polyimide layer with an electron beam;        forming a second polyimide layer on said core layer and removing said substrate, thereby forming a polyimide film having a two-layer structure; and        bonding said second polyimide layer as a lower cladding to another substrate, thereby forming a core having a predetermined shape in said core layer.        
Also, it is provided a method of manufacturing a polyimide optical waveguide, comprising steps of:                forming a first polyimide layer on a substrate;        forming a second polyimide layer on the first polyimide layer, the second polyimide layer having a refractive index higher than that of the first polyimide layer,        forming a third polyimide layer on the second polyimide layer, the third polyimide layer having a refractive index lower than that of the second polyimide layer; and        performing electron beam lithography on the three polyimide layers, thereby forming a core in the second polyimide layer, the core having a predetermined refractive index and a predetermined shape.        
The use of electron beam irradiation for changing the refractive index of a polymer or for performing a lithography yields some disadvantages. The electrons must be drained once they passed through the polymer, thus it is necessary to provide the polymer layer with a conductive substrate (e.g. of silicon, as in the above mentioned patents), which is to be removed for further processing. As a consequence of the use of said conductive layer, there may be a step of separation of the irradiated polymeric layer, said separation being critical in view of the thin thickness of the layer (in the order of μm).
J. W. Jeong et al., Journal of the Korean Physical Society, Vol. 39, No. 2 (2001), 250-254 disclose a photodefinable polyimide as useful in passive photonic devices, according to a method of preparation as from the following scheme:

(wherein R is —CH2CH2—OCO—C(CH3)═CH2; no specific meaning is provided for Ar and X). A soluble aromatic polyimide precursor is spin-coated, suitably photomasked and exposed to UV radiation to form a cross-linked polyimide precursor. After development, the cross-linked precursor is subsequently converted to fully imidized polyimide by thermal curing.